With the recent development of information communication technology, a variety of wireless communication technology has been developed. A WLAN is the technology permitting wireless access to Internet in home or companies or specific service areas by the use of portable terminals such as a personal digital assistant (PDA), a laptop computer, and a portable multimedia player (PMP) on the basis of a radio frequency technology.
The IEEE (Institute of Electrical and Electronics Engineers) 802 which is a standardization of the WLAN technology established in February, 1980 has carried out much standardization work. In the initial WLAN technology, a data rate of 1 to 2 Mbps was supported by the use of frequency hopping, spread spectrum, and infrared communication using a frequency of 2.4 GHz in accordance with the IEEE 802.11. In recent years, 54 Mbps in maximum can be supported by the use of the orthogonal frequency division multiplex (OFDM) technology to the WLAN. In addition, the IEEE 802.11 has developed or is developing a variety of technical standards for improvement in quality of service (QoS), compatibility of an access point (AP) protocol, security enhancement, wireless resource measurement, wireless access in vehicular environment, fast roaming, wireless mesh network, inter-working with external networks, wireless network management, and the like.
Among the IEEE 802.11 standards, IEEE 802.11b uses a frequency band of 2.4 GHz and supports the maximum communication rate of 11 Mbs. IEEE 802.11a commercialized after IEEE 802.11b uses not the frequency band of 2.4 GHz but a frequency band of 5 GHz to thereby lessen effect of interference as compared with that of the significantly confused frequency band of 2.4 GHz, and employs OFDM technology to thereby enhance the maximum communication rate up to 54 Mbps. However, it is disadvantageous that IEEE 802.11a has a shorter communication distance than IEEE 802.11b. Meanwhile, IEEE 802.11g uses the frequency band of 2.4 GHz like IEEE 802.11b but supports the maximum communication rate of 54 Mbps. Further, IEEE 802.11g has attracted a lot of attention since it satisfies backward compatibility, and is superior to IEEE 802.11a in terms of the communication distance.
Also, to overcome a limit to the communication rate that has been pointed out as a weak point in a WLAN system, there is IEEE 802.11n established relatively recently. IEEE 802.11n is intended to enhance the rate and reliability of the network, and extend an operation distance of a wireless network. More specifically, IEEE 802.11n supports a high throughput (HT) with the maximum data processing rate of 540 Mbps or more, and is based on multiple inputs and multiple outputs (MIMO) technology that uses multiple antennas at both a transmitter and a receiver to minimize a transmission error and optimize the data rate. Further, this standard may not only employ a coding method of transmitting several duplicate copies to improve data reliability, but also employ the OFDM technology to increase the rate.
As the WLAN is actively spread and applications using the WLAN become various, a new WLAN system for supporting a throughput higher than the data processing rate supported by IEEE 802.11n has recently been on the rise. However, an IEEE 802.11n medium access control (MAC)/physical layer (PHY) protocol is not effective in providing a throughput of 1 Gbps or more. The reason is because the IEEE 802.11n MAC/PHY protocol is dedicated to operate a single station (STA), i.e., an STA having one network interface card (NIC) and thus overheads are additionally increased as a throughput of a frame increases while keeping the existing MAC/PHY protocol of IEEE 802.11n intact. In conclusion, there is a limit to improvement in the throughput of the wireless communication network while keeping the existing IEEE 802.11n MAC/PHY protocol having the single STA architecture, intact.
Accordingly, to achieve the data processing rate of 1 Gbps or more in the wireless communication network, a new system different from the existing IEEE 802.11n MAC/PHY protocol having the single STA architecture is required. As the next version of the IEEE 802.11n WLAN system, a very high throughput (VHT) WLAN system is one of the IEEE 802.11 WLAN systems that have recently been newly proposed to support the data processing rate of 1 Gbps or more in an MAC service access point (SAP). The term “VHT WLAN system” is arbitrary, and a practicality test is being currently applied to the VHT WLAN system using 4×4 MIMO and a channel bandwidth of 80 MHz in order to provide the throughput of 1 Gbps or more.
In the VHT WLAN, two methods of using a channel of less than 6 GHz and using a channel of 60 GHz are being discussed as a method for achieving the throughput of 1 Gbps or more. Here, the reference of the throughput, i.e., 1 Gbps or more is a value measured in the MAC SAP. The channel or less then 6 GHz shows relatively broad service coverage, but has a disadvantage that an available channel bandwidth is small. On the other hand, the channel of 60 GHz has an advantage that the available channel bandwidth is large, but has relatively narrow service coverage as compared with that of the channel of less than 6 GHz due to the nature of the channel.
As a method of efficiently using the advantage and disadvantage of the two channels, one among recently proposed methods is an overlay WLAN. In the overlay WLAN, the channel of 60 GHz is used when near stations communicate with each other, and the channel of less than 6 GHz is used when distant stations communication with each other. Such an overlay WLAN achieves not only the high throughput by using a relatively large channel bandwidth in the channel of 60 GHz when the stations are located near to each other, but also stable communication by using the channel of less than 6 GHz when the stations are located apart from each other even if the throughput is lowered a little.
FIG. 1 shows topology of a VHT WLAN system for explaining such an overlay WLAN.
Referring to FIG. 1, the VHT WLAN system may include one or more AP VHT STAs, and one or more non-AP VHT STAs. The AP VHT STA may have a multi-radio interface (VHTL6 and VHT60) to support both the channel of less than 6 GHz and the channel of 60 GHz. Further, the non-AP VHT STA may have a dual-band interface for each of the channel of less than 6 GHz and the channel of 60 GHz. However, since the non-AP VHT STA has only a single-radio interface, it cannot support both the channel of less than 6 GHz and the channel of 60 GHz.
As shown in FIG. 1, in the case of the AP VHT STA, the service coverage in the channel of 60 GHz (VHT60 coverage) is relatively narrow, but the service coverage in the channel of less than 60 GHz (VHTL6 coverage) is relative broad. In this case, if the non-AP VHT STA is located within the VHT60 coverage, a relatively high throughput can be achieved because the non-AP VHT STA can use a channel band of 60 GHz in communicating with the AP VHT STA. On the other hand, if the non-AP VHT STA moves from the VHT60 coverage to the VHTL6 coverage, the non-AP VHT STA cannot use the channel band of 60 GHz and has to switch to the channel band of less than 6 GHz so as to communicate with the AP-VHT STA. Thus, if the non-AP VHT STA is located outside the VHT60 coverage, it is difficult to achieve the high throughput since the non-AP VHT STA cannot use the channel band of 60 GHz in communicating with the AP VHT STA.
Besides, if the non-AP VHT STA is located outside the VHT60 coverage, it has to use not the channel band of 60 GHz but the channel band of 6 GHz when communicating with other neighbor non-AP VHT STAs in an ad-hoc mode as well as when communicating with the AP VHT STA in an infrastructure mode. The reason is because the non-AP VHT STA has only the single-radio interface and cannot support both the channel of less than 6 GHz and the channel of 60 GHz. This causes the AP VHT STAs to inevitably use the channel band of less than 6 GHz even though the non-AP VHT STAs are located within the VHTL6 coverage and capable of communicating with each other through the channel band of 60 GHz in the ad-hoc mode, thereby lowering the efficiency of the overlay WLAN in the VHT WLAN system.